1. Technical Field
The present invention relates to a radio transmitting apparatus and radio transmitting method.
2. Description of the Related Art
Mobile communication applies ARQ (Automatic Repeat reQuest) to downlink data from a wireless communication base station apparatus (hereinafter abbreviated as “base station”) to a wireless communication mobile station apparatus (hereinafter abbreviated as “mobile station”). That is, the mobile station feeds back an ACK/NACK signal showing an error detection result of downlink data to the base station. The mobile station performs a CRC check of downlink data, and, if CRC=OK (i.e., no error), feeds back an ACK (Acknowledgement) to the base station, or, if CRC=NG (i.e., error present), feeds back a NACK (Negative Acknowledgement) to the base station. This ACK/NACK signal is transmitted to the base station using an uplink control channel such as a PUCCH (Physical Uplink Control Channel).
Further, the base station transmits control information for indicating a downlink data resource allocation result, to the mobile station. This control information is transmitted to the mobile station using a downlink control channel such as L1/L2CCHs (L1/L2 Control Channels). Each L1/L2CCH occupies one or a plurality of CCEs (Control Channel Elements). In case where one L1/L2CCH occupies a plurality of CCEs, one L1/L2CCH occupies a plurality of consecutive CCEs. According to the number of CCEs required to report control information, the base station allocates one of a plurality of L1/L2CCHs, to each mobile station, and maps control information on the physical resources associated with the CCEs occupied by each L1/L2CCH and transmits control information.
Further, to associate CCEs and PUCCHs for efficient use of downlink communication resources is being studied. According to this association, each mobile station can decide the PUCCH number to use to transmit an ACK/NACK signal from each mobile station, based on the CCE number associated with the physical resources in which that control information for that mobile station is mapped.
Further, as shown in FIG. 1, to code-multiplex a plurality of ACK/NACK signals from a plurality of mobile stations by spreading using ZC (Zadoff-Chu) sequences and Walsh sequences (see Non-Patent Document 1) is being studied. Note that the sequence length of a pure ZC sequence is a prime number, and therefore a pseudo ZC sequence of a sequence length of 12 is generated by cyclically extending part of the ZC sequence of a sequence length of 11. Also, note that a pseudo ZC sequence will also be referred to as a “ZC sequence” below for ease of explanation. In FIG. 1, (W0, W1, W2 and W3) represents a Walsh sequence of a sequence length of 4. As shown in FIG. 1, a mobile station first performs first spreading of an ACK or NACK in an SC-FDMA symbol using a ZC sequence (having a sequence length of 12) in the frequency domain.
Next, the ACK/NACK signal after the first spreading is subjected to an IFFT (Inverse Fast Fourier Transform) according to W0 to W3. The ACK/NACK signal spread using a ZC sequence of a sequence length of 12 in the frequency domain is transformed into a ZC sequence of a sequence length of 12 in the time domain by this IFFT. Then, the signal after the IFFT is further subjected to second spreading using the Walsh sequence (having a sequence length of 4). That is, one ACK/NACK signal is mapped over four SC-FDMA symbols. Similarly, other mobile stations spread ACK/NACK signals using ZC sequences and Walsh sequences.
Note that different mobile stations use ZC sequences of different cyclic shift amounts in the time domain or different Walsh sequences. Here, the sequence length of the ZC sequence in the time domain is 12, so that it is possible to use twelve ZC sequences with cyclic shift amounts of 0 to 11 generated from the same ZC sequence. Further, the sequence length of a Walsh sequence is 4, so that it is possible to use four different Walsh sequences. Consequently, it is possible to code-multiplex ACK/NACK signals from maximum 48 (12×4) mobile stations in the ideal communication environment.
ACK/NACK signals from other mobile stations are spread using ZC sequences of different cyclic shift amounts or different Walsh sequences, so that the base station can separate ACK/NACK signals from mobile stations by performing despreading using a Walsh sequence and correlation processing of ZC sequences. Further, as shown in FIG. 1, block spreading codes of a sequence length of 3 is used for RSs (Reference Signals). That is, RSs from different mobile stations are code-multiplexed using second spreading sequences of a sequence length of 3. By this means, RS components are transmitted over three SC-FDMA symbols.
Here, the cross-correlation between ZC sequences of different cyclic shift amounts generated from the same ZC sequence is virtually 0. Consequently, in the ideal communication environment, as shown in FIG. 2, a plurality of ACK/NACK signals code-multiplexed using ZC sequences of different cyclic shift amounts (cyclic shift amounts of 0 to 11) can be separated in the time domain by correlation processing in the base station without inter-code interference.
However, due to various influences such as transmission timing lags in mobile stations, multipath delay waves and frequency offset, a plurality of ACK/NACK signals from a plurality of mobile stations do not always arrive at the base station at the same time. For example, as shown in FIG. 3, in case where the transmission timing for an ACK/NACK signal spread using a ZC sequence of a cyclic shift amount of 0 is delayed from the right transmission timing, the correlation peak of the ZC sequence of a cyclic shift amount of 0 appears in the detection window for the ZC sequence of a cyclic shift amount of 1. Further, as shown in FIG. 4, in case where an ACK/NACK signal spread using a ZC sequence of a cyclic shift amount of 0 produces a delay wave, interference due to this delay wave leaks and appears in the detection window for the ZC sequence of a cyclic shift amount of 1. That is, in these cases, the ZC sequence of a cyclic shift amount of 0 interferes with the ZC sequence of a cyclic shift amount of 1. Therefore, in these cases, performance of separating an ACK/NACK signal spread using a ZC sequence of a cyclic shift amount of 0 and an ACK/NACK signal spread using a ZC sequence of a cyclic shift amount of 1 deteriorates. That is, if ZC sequences of consecutive cyclic shift amounts are used, there is a possibility that the performance of separating ACK/NACK signals deteriorates. To be more specific, although there is a possibility that interference due to transmission timing lags occurs together with interference from a cyclic shift amount of 1 to a cyclic shift amount of 0 and interference from a cyclic shift amount of 0 to a cyclic shift amount of 1, as shown in the figure, the influence of a delay wave only produces interference from a cyclic shift amount of 0 to a cyclic shift amount of 1.
Therefore, conventionally, in case where a plurality of ACK/NACK signals are code-multiplexed by spreading using ZC sequences, enough cyclic shift amount differences (i.e., cyclic shift intervals) are provided between ZC sequences to prevent inter-code interference from occurring between ZC sequences. For example, assuming that the difference in the cyclic shift amount between ZC sequences is 2, ZC sequences of six cyclic shift amounts of 0, 2, 4, 6, 8 and 10 in twelve cyclic shift amounts of 0 to 11 are used for first spreading of ACK/NACK signals. Consequently, in case where ACK/NACK signals are subjected to second spreading using Walsh sequences of a sequence length of 4, it is possible to code-multiplex ACK/NACK signals from maximum 24 (6×4) mobile stations. However, there are only three patterns of RS phases, and therefore only ACK/NACK signals from 18 mobile stations can actually be multiplexed.    Non-Patent Document 1: “Multiplexing capability of CQIs and ACK/NACKs form different UEs,” 3GPP TSG RAN WG1 Meeting #49, R1-072315, Kobe, Japan, May 7-11, 2007